1. Field of the Invention
This invention relates to a process for producing a toner having a good electrophotographic performance, having toner particles in which a fine-particle colorant stands finely and uniformly dispersed. More particularly, this invention relates to a process for producing a toner by polymerizing polymerizable monomers in an aqueous medium to form toner particles; the monomers being present in particles of a polymerizable monomer composition in which a fine-particle colorant stands finely and uniformly dispersed.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are known as electrophotography. In general, using a photoconductive material, copies or prints are obtained by forming an electrostatic latent image on a photosensitive member, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and thereafter fixing the toner image onto the transfer medium by the action of heat, pressure, heat-and-pressure or solvent vapor. As methods for developing the electrostatic image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed, and methods suited for individual image-forming processes are employed.
Toners used for such purpose have been commonly produced by melt-kneading colorants comprising dyes or pigments, into thermoplastic resins to effect uniform dispersion, followed by pulverization using a fine grinding mill and classification using a classifier to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a production method, but there is a limit to the range in which toner materials are selected. For example, colorant-dispersed resins must be brittle enough to be pulverizable with ease by means of a fine grinding mill economically available. Such colorant-dispersed resin having been made brittle tend to result in a broad particle size distribution of the particles formed when actually pulverized at a high speed, especially causing a problem that relatively large particles tend to be produced. Moreover, such highly brittle materials tend to be further pulverized in developing assemblies of copying machines or the like. Also, it is difficult to uniformly disperse solid fine particles of colorants or the like in the resin, and some toners cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency, depending on the degree of dispersion. Also, colorants may come bare to rupture sections of pulverized particles, and may cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, toners produced by suspension polymerization, toners produced by various polymerization and their production processes are disclosed in Japanese Patent Publications No. 36-10231, No. 43-10799, No. 51-14895 and so forth. For example, in the process for producing toners by suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in a continuous phase (e.g., an aqueous medium) containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization reaction to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization, the toner particles are not required to be brittle, and hence soft materials can be used as the resin. Also, colorants do not come bare to the surfaces of toner particles, and hence the toner particles having a uniform triboelectric charging performance can be obtained. This method has such advantages. Also, since the toner particles obtained have a relatively sharp particle size distribution, the step of classification can be omitted, or, even when classified, toner particles can be obtained in a high yield. The method also has the advantage that, since a low-softening substance can be encapsulated in toner particles in a large quantity, the toner particles obtained have a superior anti-offset properties.
When the monomer mixtures containing a fine-particle colorant are obtained, it has been common to use a DYNO-MILL brand dispersion mill, having been made commercially available from Shinmaru Enterprises Corporation as disclosed in Japanese Patent Application Laid-Open No. 6-11900; or a COBAL-MILL brand dispersion mill, manufactured by Shinko Pantec Co., Ltd. as disclosed in Japanese Patent Application Laid-Open No. 6-75429.
A cross-section of the main body of the DYNO-MILL brand dispersion mill and a diagram of a system incorporated with the main body are shown in FIG. 10. Reference numeral 121 denotes a main-body casing; 122, a material feed opening; 123, a discharge opening; 124, an agitator shaft; 125, media; 126, a circulating pump; 127, a holding tank; 128, agitator disks; and 129, a motor. A dispersion target liquid material put into the holding tank 127 is fed through the material feed opening 122 by means of the circulating pump 126, passed through the layers of media 125 repeating their motion by a rotational force of the agitator disks 128 to effect dispersion, then separated from the media, thereafter discharged through the discharge opening 123, and returned to the holding tank 127. This cycle is repeated to carry out dispersion.
A cross-section of the main body of the COBALL-MILL brand dispersion mill and a diagram of a system incorporated with the main body are shown in FIG. 11. Reference numeral 231 denotes a main-body casing; 232, a material feed opening; 233, a discharge opening; 234, a media separator; 235, media layers; 236, a circulating pump; 237, a holding tank; and 238, a rotor. A dispersion target liquid material put into the holding tank 237 is fed through the material feed opening 232 by means of the circulating pump 236, passed through the media layers 235 repeating their motion by a rotational force of the rotor 238 to effect dispersion, then separated from the media by the media separator 234, thereafter discharged through the discharge opening 238, and returned to the holding tank 237. This cycle is repeated to carry out dispersion.
However, the use of such a conventional media type dispersion machine to obtain the fine-particle-colorant-containing monomer mixture may remarkably cause "packing", which is a phenomenon where the media is pushed outward against the wall when the centrifugal force of the rotor is stronger than the flow of the monomer mixture, because the direction in which the monomer mixture flows is not in agreement with the direction of centrifugal force. This makes non-uniform the media-packed layers in the machine. As the result, the flow of the monomer mixture tends to pass through the part where the media layers have become thin to have a small flow resistance, bringing about a problem that the dispersion efficiency lowers greatly.
When on the other hand the centrifugal force of the rotor is weaker than the flow of the monomer mixture, the media is inevitably carried away toward the discharge opening because of the flow of the monomer mixture to cause a phenomenon of "blocking" of media in the vicinity of the discharge opening, making it difficult to operate the machine stably. Thus, it has been difficult to keep a balance between the centrifugal force caused by the rotation of the rotor and the flow of the monomer mixture so as to stably produce the fine-particle-colorant-containing monomer mixture.
In addition, a decrease of image density may occur when images are formed using a toner having toner particles obtained using a fine-particle-colorant-containing monomer mixture in which a colorant and a charge control agent stand dispersed insufficiently. There is a problem that granulation performance tend to lower when such an insufficiently dispersed fine-particle-colorant-containing monomer mixture is used to carry out granulation in an aqueous medium containing a dispersion stabilizer. Accordingly, in order to obtain a well dispersed fine-particle-colorant-containing monomer mixture by means of the media type dispersion machine, it is necessary to improve media efficiency. In order to improve the media efficiency, media having a small diameter, stated specifically, media of 2 mm or smaller in diameter may be used, whereby the media efficiency is greatly improved. However, the use of the media of 2 mm or smaller in diameter results in a high packing to more tend to cause the above phenomena of packing and blocking, thus it has been difficult to operate the machine stably and also difficult to produce the fine-particle-colorant-containing monomer mixture stably.
In order to obtain a well dispersed fine-particle-colorant-containing monomer mixture by means of the conventional media type dispersion machine while relieving the phenomena of packing and blocking, it is preferable to use beads larger than 2 mm, and more preferably beads of 3 mm or larger. This, however, has taken a long timer to achieve the intended dispersion.